1. Field of the Invention
The present invention relates to a circuit for driving a solenoid valve, and more particularly to a solenoid valve driving circuit which is capable of driving a solenoid valve at high speed.
2. Description of the Prior Art
In general, to carry out the high speed operation of a solenoid valve, a driving current with a suddenly standing-up leading edge is required. This is because of the maintenance current used for operating the solenoid valve. In the prior art, specifically in Japanese Patent Public Disclosure No. 109864/80, there is proposed a solenoid valve driving circuit in which the value of the maintenance current is kept at a level just below that required to start operation of the solenoid valve, this level being maintained regardless of any change in the voltage of the powder source or the like, so as to carry out the operation of the solenoid valve at high speed.
However, in order to keep the maintenance current at a predetermined level, the proposed circuit requires a detecting resistor for detecting the level of the maintenance current, a feedback circuit for feeding back the result of the detection by the detecting resistor and other complicated circuitry. Furthermore, the optimum level of the maintenance current varries in accordance with the temperature of the solenoid valve so that in such an arrangement where the level of the maintenance current is kept at a predetermined constant level, it is necessary to design the circuit to allow for a certain amount of variation in the maintenance current from the optimum level. Therefore, it is difficult to maintain the operation of the solenoid valve under optimum condition at all times.